When you're talking about PG, labels are important--the needs of a PG kid are greater--but the concept of a cut off score is not particularly useful.
When public schools evaluate a student to see if the student has a disability, they have to consider confidence intervals, observation of student behaviors at school and at home, as part of a variety of sources of information. They have to construct a plan and a program that meets the educational needs of the person, not the category. Certainly the same principles should apply to gifted education.
Dr. Deborah Ruf's book Five Levels of Gifted is rich in qualitative descriptions of the developmental milestones of gifted students. If I were trying to figure out if my child was PG, I'd skip the score tables and read that book.
